Separating particles in large volumes is a significant problem in many industries. Separation is desirable when one or more types of the particles has commercial value. Separation is also used when a mixture of particles having one environmentally undesirable component must be discarded in an expensive manner. In such a situation, it is desired to separate the environmentally undesirable particles from the other particles so that the volume of material that must be expensively discarded may be reduced. One example of a use for a process that separates commercially-valuable particles from other particles is separating gypsum from fly ash in a flue gas desulfurization process.
Burning coal to create electricity is one of the main sources of electrical power in the United States. In the past, the byproducts produced from burnt coal were exhausted from a smokestack into the atmosphere. As public awareness grew about the harmful environmental effects of such practices, the public demanded that the exhaust from power plants be cleaned prior to being emitted from a smokestack.
A flue gas desulfurization process is a common cleaning process used in coal-fueled power plants. One drawback to the process is that a large quantity of fly ash-contaminated hydrated calcium sulfate is produced as a byproduct of the process. Hydrated calcium sulfate is commonly referred to as gypsum and is commonly used to manufacture plaster of paris and wall board. Disposing of the fly ash-contaminated gypsum is a problem for power companies that significantly increases the expense of cleaning the exhaust. It has thus been desired in recent years to find uses for the byproducts of the flue gas desulfurization process and other cleaning processes in order to offset the costs of the cleaning process.
Practical uses for the gypsum byproduct produced during a flue gas desulfurization process include using the gypsum to form plasters and to fabricate wall board. Unfortunately, the gypsum extracted directly from many flue gas desulfurization processes is not immediately commercially useable because it is contaminated with a relatively large amount of fly ash. Although fly ash does not significantly alter the structural properties of the gypsum, the fly ash darkens the color of the gypsum causing it to lose its commercial value. More significantly, fly ash reduces adhesion of the paper to the board and also increases board weight. It is thus desired in the art to provide a method and apparatus for efficiently removing fly ash from relatively large quantities of gypsum.
Known methods for separating fly ash from gypsum produced in flue gas desulfurization systems utilize hydroclones, screens, or hydroseparators. Gypsum is commercially desirable for wall board applications only when it has a purity of 92 percent and above. To date, the methods and apparatus known in the art for separating wall board-quality gypsum from the byproducts of flue gas desulfurization systems have not economically and consistently achieved wall board-grade gypsum.
One such known system uses a hydroclone to separate the fly ash from the gypsum. A hydroclone system uses pump discharge pressure to accelerate particles in the hydroclone. The thickened, coarse gypsum particles are pushed to the circumference of the hydroclone by centrifugal force and are concentrated in the underflow while the smaller particles and water move to the axis of the hydroclone where they are removed in the overflow. The high shear rates and the low residence time in the hydroclone environment do not effectively separate the particles causing the underflow to be contaminated with the fine fly ash particles. The overflow also contains an undesirable quantity of the gypsum. Adding hydroclones in series increases the separation quality but also increases the expense of fabricating and operating the system.
Screens have also been used to separate fine particles from coarse particles in applications such as separating fly ash from gypsum. Screens are difficult to effectively employ when the difference in particle size is small and when the particles themselves are small. In the case of separating fly ash from gypsum, appropriately-sized screens are prone to clogging and are subjected to an undesirable amount of abrasive forces. It has also been found that a large number of screens are necessary to adequately separate gypsum from fly ash. In addition to the other problems with the screens, the number of screens impractically increases the expense of the system.
Other separation systems include the use of other hydroseparator designs that separate particles based on differences in settling characteristics. Such devices have various problems that make them inefficient, including difficulties in achieving uniform distribution of the rising liquor and/or wash water and inadequate dampening of the kinetic energy of the feed slurry. Some of these devices are also incapable of efficiently displacing mother liquor containing fines from the coarser particles. Separation systems that use flotation require the added expense of flotation agents and the high energy cost of supplying air for froth formation. In addition, there must be a chemical difference between the surfaces of the fines and coarse particles for the flotation agents to work effectively. It is thus desired in the art to provide a method and apparatus for separating gypsum from fly ash such that the resulting separated gypsum is pure enough to use in a wall board fabrication operation.
Another situation where it is desirable to separate particles is in the recovery of clay. The desirable clay is typically contaminated with grit. In known recovery processes, a significant amount of clay remains in the grit resulting in a loss of about 10% of the clay product. It is desired in the art to provide a method and apparatus for separating the clay from the grit that is more efficient.
The method and apparatus of the invention should thus be capable of separating particles with slow settling characteristics from particles with fast settling characteristics in a wide variety of applications. The desired system must be efficient, effective, substantially maintenance-free, inexpensive to operate, and easily adjustable to accommodate different flow rates and concentrations of different types of particles.